Power Unit

According to this http://en.community.dell.com/support-forums/desktop/f/3514/t/19267468.aspx pretty much any standard ATX power supply will fit but the XPS 420 needs 6 SATA power outputs. If your selected PSU does not have all of these you can buy adapters for the standard IDE power connectors.

The physical size of your PSU, some are hard to replace due to being a non standard size.

The amount of power need from the PSU don't skimp.
3 The correct connections for your equipment

Physical Dimensions

Besides the specs and form factors, the physical dimensions are also important factors in selecting a compatible power supply. Here is an outline of the physical dimensions of most standard power supplies:

ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.
Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.
MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.
Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.

Use the data above to determine if a particular power supply would fit your case.

The quality of a power supply can be estimated by its weight. While this is not a true scientific or thorough measurement of the power supply reliability, it is nevertheless a very simple and easy way for ordinary PC users to estimate and compare the quality of a power supply. Why weight matters http://www.directron.com/psu.html

Power supply calculator http://www.antec.outervision.com/

Correct connections
Some boards have 20 pin connectors others 24 pin
There is often a 4 pin plug required to power Intel CPUs
Molex D plugs for IDE HDD and CD/DVD drives
SATA power connections for latest HDDs and DVD drives.